What is this Fresnel node?
Real objects both reflect and adsorb light. How much reflection you see at each point of the object depends on the angle at which you look at it.  Think of a lake of clear water.  Up close, none of the water is reflected, so you see to the bottom. Farther away, the water reflects more, so you see less into the water and more of what is reflected by the water.

The fresnel node applies this effect.  There are several good tutorials on Physics Based Rendering (PBR) that talk about Fresnel effect and using the Fresnel node.  I recommend a two part tutorial by Blender Guru. Part 1 contains a pointer to part 2.